(1) Field of the Invention
This invention relates to an electrical insulating oil and oil-filled electrical appliances that are impregnated with the insulating oil. More particularly, the invention relates to an insulating oil and oil-filled electrical appliances in which the insulating oil is composed of a miture of diarylalkanes and alkyl-3-arylindane derivatives and/or 1,3-diarylbutene derivatives. The term "insulating oil" herein referred to includes dielectric fluids.
(2) Description of the Prior Art
Electrical appliances such as capacitors, power cables and transformers have recently been made to withstand high electric voltages while made small in size. With this tendency, synthetic resin films or the like are used in place of or together with the conventional insulating paper. As a result, the properties required of electrical insulating oils have become quite severe.
In the conventional art, mineral oils, alkylbenzenes, polybutenes, alkylnaphthalenes, alkylbiphenyls and diarylalkanes have been proposed and used as electrical insulating oils for oil-filled electrical appliances. However, the performance of these insulating oils do not match the above-mentioned development in electrical appliances. In particular, the properties of such insulating oils are not satisfactory with regard to electrical appliances employing insulating materials made of synthetic resins.
With the requirement of high-voltage resistance and size reduction, it is necessary that the electrical insulating oils have a high dielectric breakdown voltage, low dielectric loss tangent, and good hydrogen gas absorbing capacity.
The hydrogen gas absorbing capacity indicates the stability of the insulating oil against corona discharge under high electric voltage conditions. The higher the gas-absorbing capacity, the smaller the likelihood of corona discharge, which leads to the advantage of the insulating oil having excellent stability or durability.
Meanwhile, in order to meet the requirement of high voltage use, plastic films such as polyolefin films, polystyrene films and polyester films are used in place of or as a part of the conventional insulating paper as insulating materials, or dielectric materials for electrical appliances such as oil-filled electric cables and capacitors. In view of their dielectric strength, dielectric loss tangent and dielectric constant, the polyolefin films, especially, polypropylene and cross-linked polyethylene films are preferred as the plastic films.
When these polyolefin films are impregnated with an insulating oil, some oils cause the films to swell to some extent. If a film becomes swollen, the thickness of insulating layer increases, as a result, the resistance to the insulating oil flow increases in cables, and insufficient impregnation of insulating oil occurs in electric capacitors, causing the formation of void spaces (unimpregnated portions) and the undesirable lowering of the corona voltage.
In connection with the above-mentioned conventional electrical insulating oils, the values of the dielectric breakdown voltages (BDV) and the dielectric loss tangents (tan .delta.) are satisfactory to some extent, but the hydrogen gas absorbing capacity or corona discharge characteristics and the size stability in connection with polypropylene films are not satisfactory.